I was the only person to have anything close to a scientific answer based on my extensive research in the growing field of "things I sort of remember". In other words, I am an expert Moltencoreologist.

So the answer: Tall bosses. Big rooms.

These are both essential. Short bosses in very big rooms don't work. Those are called gnomes and they are utterly unmemorable except if you kill thousands at a time.

Conversely, big bosses in small rooms don't work. Note that this is relative. While Magtheridon's room is big and he is big, he is too big for the room.

A cluttered room can simulate the effects of a small room. For example, Kel'Thuzad's room is filled up with a giant throne and the walls are filled with hostile undead, reducing the effective space. Furthermore, his small footprint means that melee are packed up, reducing the apparent size for them. That other KT, Kael'thas, has rooms scattered with banners, statues, and strange glowing boxes, which also reduce the perceived size. This is in addition to him ultimately dying in what appeared to be a large storage closet.

We've seen all the negative examples. What about fights that worked?

Raganaros tops the list. He is very tall. And wide. The room he's in is gigantic. The size of the room has the benefit of allowing players to stand far enough back to see his head.

By this point you may be wondering, is this just another silly post? Yes. No. Sort of. I'm trying to find something universal here. Something applicable. Challenging fights are dependent on the skill of the players, their gear, and not getting nerfed two months later. And they can't be replicated. Once you know a fight, you know a fight. Challenge tends to be a brick wall which is suddenly gone, without the slightest hint that it was ever there. It cannot instill feelings if "you weren't there then". Silly fights, such as the pygmy in Lost City, don't retain their value. The joke gets old. But maybe a tall boss in a big room has some lasting power. It is not a brilliant design or a substitute for a fun, challenging fight, but as a general rule, the last boss should be tall and in a big room. A well-decorated room.

- is big, - has a credible reason to be big (Ragnaros has, Arthas doesn't),- although he is big you can still see him, at least as ranged DD/healer,- has a credible reason to be where he is and- has a credible way to actually have come to where he is (how is that boss to fit through THAT door??)

In short: The bosses should be big and thus feel powerful. And in a addition to that the player should not constantly have the feeling that the designers consider him stupid. Oh, and I hate it if all I see is his one toe.

Very valid point, Nils... Credible reason to be big. What is Arthas? An undead Vyrkul? This is why Shadowfang Keep is my favorite instance, because its scale is small, and it makes sense to be the way that it is.

There's definitely something about Raggy. Every time I visited him in MC I couldn't help but feel this urge to take a screenshot, because even though he was level 60 and getting pwned by three level 80s, there was something about his size that just made him feel awe-inspiring.

My favourite boss fight in WOTLK was Yogg-Saron, who is quite big as well (and you get a feeling that there's a lot more of him underground).

I loathe small bosses (can't see them), small rooms (can't rotate camera with a door or wall leaping into your face) and dragons when there's environmental effects to move from (STUPID WINGS STOP BLOCKING MY VISION).

But in all seriousness, I agree completely. The first time I queued for Ahune, the Midsummer Fire Festival boss, even though he was "just" a seasonal boss, his scale and the enormity of the cavern he emerged in was very impressive. In comparison, another seasonal boss, Coren Direbrew, is just a runt in a corner.